This invention relates to fibers and composite fibers, of wholly aromatic polyamides, a process for production thereof, and uses of these fibers.
Wholly aromatic polyamides of the poly(m-phenylene isophthalamide) type (to be sometimes abbreviated "PMIA" hereinafter) have superior heat resistance and fire retardancy because they have a glass transition point of about 280.degree. C., a melting point and a heat decomposition point each of about 420.degree. C. and a limiting oxygen index of-about 30. Furthermore, they have moderate rigidity. For this reason, these aromatic polyamides have been produced and marketed in quantities as fibers under registered trademarks Nomex.RTM. (Du Pont), Conex.RTM. (Teijin Limited), etc. These commercial fibers are known to be produced by, for example, the wet method or dry method described in Japanese patent Publication Nos. 870/1963, 50219/1972 and 38612/1971 and U.S. Pat. No. 3,360,598, or the dry jet-wet spinning method described in Japanese patent publication No. 815/1967, and in any case, their production relies on these "solution spinning" methods. The primary reason for it is that since PMIA has a high melting point which is close to its heat decomposition point, its melt-spinning is very difficult.
The high cost of production owing to the low productivity and the need for investment in solvent recovery or neutralization equipment may be cited as the disadvantage of the solution spinning method. It also has difficulties that cannot be ignored and are required to be overcome.
Firstly, large-denier fibers (bristles) having a size of at least 30 denier, particularly at least 50 denier, are very difficult to produce by the solution spinning method. In the solvent removing step in the solution spinning process, the solvent at the outer skin portion of the fibers generally dissipates preferentially and the skin portion begins to be coagulated first. Hence, as the fibers increase in denier size, the solvent removal of the core portion is gradually retarded. Thus, to produce large-denier fibers, the solvent removing step must be performed for an extraordinarily long period of time, and it is difficult in practice to produce such fibers. In addition, a great difference arises in microstructure between the surface and the inside of the resulting fibers owing to a difference in solvent removal, and the fibers assume an extreme skin-core structure and becomes almost useless. In fact, as far as the present inventor knows, wholly aromatic polyamide fibers having a size of more than 50 denier are not commercially available.
A second point is that the organic solvent and the inorganic salts as a dissolving aid used in solution spinning remain in the final fibers. Aprotic polar solvents such as dimethylformamide, dimethylacetamide or N-methylpyrrolidone are used as the solvent in the solution spinning of PMIA, and halides of metals of Group I or II of the periodic table, such as lithium chloride and calcium chloride, are used as the inorganic salts. Analysis clearly shows that such solvents and inorganic salts remain in the final product. Particularly, the fact that the solvent remains in an amount of nearly 1% causes concern not only about its adverse effects on the heat resistance of the final product, but also about the use of the final product in medical and food applications. To the best of the knowledge of the present inventor, it is difficult or substantially impossible in practice to obtain PMIA fibers completly free from the residual solvent by the solution spinning methods such as the wet or dry method. The aforesaid commercial fibers have a size of about 10 denier at the largest. But since the present invention is directed to fibers having a size of at least 50 denier which are classed as bristles, the difficulty of obtaining solvent-free fibers further increases.
A third problem of the solution spinning method is that as the denier size of a fiber produced by this method increases, its cross-sectional surface tends to become irregular. There are various uses of the bristles, but for use as a material for advanced industrial fields, the circularity of the fiber cross section is frequently regarded as an important factor. Hence, the irregular shape of the fiber cross-section is a serious problem.
The present inventor, together with coworkers, previously proposed a method of producing bristles by melt-spinning a wholly aromatic polyamide (U.S. Pat. No. 4,399,084 and European Laid-Open Patent Publication No. 0047091). This method comprises instantaneously melting a substantially solid wholly aromatic polyamide on an electrically heated thin mesh spinneret, extruding the molten polyamide through many fine openings of the mesh spinneret before it substantially loses fiber-forming ability, and immediately then solidifying the filaments by cooling while forcibly taking them up.
This method is innovative in the melt spinning of wholly aromaic polyamides, and gives wholly aromatic polyamide fibers free from a spinning solvent since it does not use the solvent. Later investigations of the present inventor have revealed some disadvantages of this method.
(1) Since the wholly aromatic polyamide is melted even instantaneously, the resulting unstretched filaments are cross-linked to such a degree that the molecular chains are difficult to orient.
(2) Since the melt-spinning is carried out using the mesh spinneret, it is difficult to stretch the filaments at such a high ratio that there is a great change in the cross-sectional area of the fibers in their longitudinal direction (therefore, when the filaments are stretched at a temperature in the vicinity of their glass transition point, the tension increases and the filaments frequently undergo breakage at those parts whose cross-sectional area is small).
(3) Since the spinning mesh spinnert, many filaments gather one on top of another as a bundle, and it is difficult to maintain a uniform dry heat temperature within a short period of time on those filaments which are located inside the bundle.
The present inventor also proposed a method of melt-spinning a wholly aromatic polyamide in which the fiber-forming polymer is fed in the form of a shaped mass to a die equipped with a spinneret having numerous closely spaced small openings such as a mesh spinneret and then spun (see U.S. Pat. No. 4,526,735 and European Laid-Open patent publication No. 0086112). The characteristic feature of this method is to feed the fiber-forming polymer to the die after it is converted into a shaped mass. The above patent documents fail to describe a stretched filaments obtained by stretching and orienting the resulting unstretched filaments.
It is an object of this invention therefore to provide novel fibers of a wholly aromatic polyamide containing at least 85 mole %, based on the entire recurring units, of m-phenylene isophthalamide units as a main component.
Another object of this invention is to provide novel wholly aromatic polyamide fibers which do not substantially contain an aprotic polar solvent, or in other words, are quite different from wholly aromatic polyamide fibers obtained by solution-spinning of a solution of a wholly aromatic polyamide in an aprotic polar solvent.
Another object of this invention is to provide wholly aromatic polyamide fibers which do not substantially contain an aprotic polar solvent and are substantially amorphous.
Another object of this invention is to provide stretchable wholly aromatic polyamide fibers which clearly show the glass transition temperature of a wholly aromatic polyamide, and when stretched at the glass transition temperature, show a very high heat shrinkage.
Another object of this invention is to provide stretched wholly aromatic polyamide fibers which are oriented but are substantially amorphous and are not crystallized.
Another object of this invention is to provide wholly aromatic polyamide fibers which have a small cross-sectional area variation in the longitudinal direction of the fibers, a substantially circular cross section, and superior toughness, bending fatigue resistance and elastic recovery.
Another object of this invention is to provide wholly aromatic polyamide fibers having a size of at least 50 denier which are very difficult to produce, or cannot substantially be produced, by solution spinning.
Another object of this invention is to provide unstretched wholly aromatic polyamide fibers which unlike conventional unstretched wholly aromatic polyamide fibers produced by solution spinning, cannot substantially be stretched in hot water.
Another object of this invention is to provide wholly aromatic polyamide fibers which unlike conventional unstretched wholly aromatic polyamide fibers produced by solution spinning or stretched products thereof, can be substantially crystallized only at temperatures near the crystallization temperature of the wholly aromatic polyamide itself.
Another object of this invention is to provide a process for producing the aforesaid wholly aromatic polyamide fibers of the invention.
Another object of this invention is to provide a brush having excellent fatigue resistance at high temperatures comprising the wholly aromatic polyamide fibers of the invention as bristles.
Further objects of this invention along with its advantages will become apparent from the following description.